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My brother and I are considering designing and building a remotely controlled submarine. As far as experience, I have some knowledge on RC systems, and I am almost done with my General license. My brother hasn't completed his license yet, but shares much of my experience.

The problem we're facing is getting a live video feed/FPV from the craft while under fresh water. We don't need great range, just enough to get down 10-15 feet vertically, maybe (though more would be much better, of course). Our goal is for it to be completely wireless, avoiding having some type of umbilical cord.

The question is, how low frequency can you go so that it is not absorbed, but intelligible enough to maintain (visual) control the craft at as low as around, say, one frame per second? I don't need amazing reaction speed like you'd need with a quadcopter.

Next would be finding what frequencies you can legally transmit this on (I think this would be classified as "remote vehicle control"?), and where to get equipment for it. (We haven't even gotten here yet, but when we do, I'd like it to stay under $500.)

Edit - so far, there are a few options I'm keeping in mind, along with others' advice (I'll accept the answer that I think most resembles the original idea):

  • Physical cable
  • Surface floating "buoy" antenna
  • long antenna to surface (similar to last option)
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  • $\begingroup$ The controller is "model craft control"; the video isn't, it's just video (unless it's sent digitally, in which case it's just data). $\endgroup$ – hobbs - KC2G Nov 21 '20 at 9:08
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    $\begingroup$ I see your point, but with the video being necessary to control the craft (as in, I lose control and risk loss if I don't have it), doesn't that make it part of control? But then, the video signal doesn't necessarily control the craft directly... Seems like a grey area to me, as I've seen similar issues arise with FPV on drones. $\endgroup$ – Galaxy Nov 21 '20 at 19:51
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    $\begingroup$ How about a 15 feet vertical (telescopic) antenna? It is not quite "surface floating" and takes care of most of your problems... $\endgroup$ – Radovan Garabík Nov 22 '20 at 8:07
  • $\begingroup$ @Randovan Garabík That's definitely an option that I'll add to the list. I imagine I could use a long wire-in-a-straw type antenna, like on many "toy" RC vehicles. The only problem is that it might limit mobility/get caught on stuff (and I'd like the submersible to be fully self-contained), but definitely very close to the original idea. You should put it in an answer. $\endgroup$ – Galaxy Nov 22 '20 at 19:10
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    $\begingroup$ @RadovanGarabík please expand your comment a bit and put it in an answer. $\endgroup$ – rclocher3 Nov 23 '20 at 17:25
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Here is some useful scholarly information about the signal attenuation (in dB) of EM waves through freshwater at various frequencies and depths. You will find a formula that you can use.

You will also need to know the antenna gains, transmitter power levels, and the receiver sensitivity at the bandwidth.

Electromagnetic wave propagation into fresh water

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    $\begingroup$ Exactly what I've been looking for, but it's making me wait 4 hours to award the bounty D: $\endgroup$ – Galaxy Nov 23 '20 at 22:16
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    $\begingroup$ Nice references! As I remembered from my Prof, propagation in fresh water isn't the real problem, it's penetrating the surface, and designing proper antennas for use under water. It seems 2.4 GHz is right out, but perhaps 48 MHz TV would be possible for 10-15 metres in fresh water. $\endgroup$ – tomnexus Nov 23 '20 at 22:53
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    $\begingroup$ @tomnexus Would you know about the legality of transmitting/broadcasting on that band? It isn't part of the ARRL bandplans. $\endgroup$ – Galaxy Nov 24 '20 at 21:40
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    $\begingroup$ @Galaxy The six meter band is close enough, 50-54 MHz. :-) $\endgroup$ – Mike Waters♦ Nov 24 '20 at 21:59
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    $\begingroup$ Yes, 48 MHz isn't an amateur band, you'd have to go to 6 metres. TV isn't allowed there anyway. I mention 48 MHz as a classic TV band, perhaps in your jurisdiction there is some allowance for low power TV transmitters (like there is for FM radio). $\endgroup$ – tomnexus Nov 24 '20 at 22:13
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SSTV could be an interesting answer here. Bandwidth requirement is about 30 KHz to reach your goal of 1 frame per second AND it's proven tech on HF bands... though you'll need to multiplex the video feed yourself (or another strategy) to use 30 KHz instead of the typical 3 KHz.

This may be a great opportunity to use a loopstick antenna. Yes for transmitting.

Typically operating temperature becomes an issue as resistive losses in the ferrite bar heat it up. Using whatever body of water you're in as a heatsink should get you several watts on 40m or even longer wavelengths, which should be enough to get through the 10-15 feet of water you're looking at.

Check out this fellow's experiments for starters.

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    $\begingroup$ A loopstick antenna in the UHF range? Note what he said about the bandwidth and frequency. $\endgroup$ – Mike Waters♦ Nov 23 '20 at 19:02
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    $\begingroup$ Hi Mike, I've just re-read the question... I don't see where he said anything about bandwidth or frequency... would you mind an explicit pointer, I'm not sure what I missed. $\endgroup$ – webmarc Nov 23 '20 at 19:40
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    $\begingroup$ It's in Galaxy's comment. I think he's right. $\endgroup$ – Mike Waters♦ Nov 23 '20 at 19:56
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Any antenna under water is creates a far more difficult problem than that of the main focus of the school project.

I would put a WiFi transmitter (ESP32, et.al., inexpensive loss if drowned) on a float above the submersible, and run a fiber optic cable to transfer image files from the FPV camera (connected to say an inexpensive Raspberry Pi Zero that won't be a major financial loss if drowned) up to the floater. That way the antenna can be up out of the water. And most of the software needed to generate and transfer image files over WiFi is already available.

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    $\begingroup$ I've considered this, but it still partially defeats the unique purpose of the vehicle - being 100% free of wires. I'll see if there are any other options first. $\endgroup$ – Galaxy Nov 21 '20 at 23:15
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    $\begingroup$ This wikipedia article: en.wikipedia.org/wiki/Radio-controlled_submarine indicates that a relatively low hobby-band frequency signal (e.g. 27MHz) can work at depths of up to 45 meters. $\endgroup$ – GreyBeardedGeek Nov 22 '20 at 19:12
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    $\begingroup$ @GreyBeardedGeek that helps. However, I believe that the problem is that, while 27MHz may work well, video transmissions require up to 6MHz of bandwidth, and are therefore restricted to 440MHz and above. We haven't even gotten to finding reasonably priced hardware for this. $\endgroup$ – Galaxy Nov 22 '20 at 22:39
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    $\begingroup$ The 6 MHz bandwidth requirement is for broadcast video at high frame rate. SSTV on HF is a 3 KHz affair. To get to the stated frame rate of 1 frame/sec, budget about 30 KHz. $\endgroup$ – webmarc Nov 23 '20 at 20:09

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